Open db in Read only mode (#588)

* Added db read only open mode and use it for the tsdb cli.

Signed-off-by: Krasi Georgiev <kgeorgie@redhat.com>

CHANGELOG.md

@@ -1,4 +1,9 @@
-## Master / unreleased
+## master / unreleased
+
+ - [FEATURE] Added `DBReadOnly` to allow opening a database in read only mode.
+    - `DBReadOnly.Blocks()` exposes a slice of `BlockReader`s.
+    - `BlockReader` interface - removed MinTime/MaxTime methods and now exposes the full block meta via `Meta()`.
+
 
  - [FEATURE] `chunckenc.Chunk.Iterator` method now takes a `chunckenc.Iterator` interface as an argument for reuse.
 

block.go

@@ -138,11 +138,8 @@ type BlockReader interface {
 	// Tombstones returns a TombstoneReader over the block's deleted data.
 	Tombstones() (TombstoneReader, error)
 
-	// MinTime returns the min time of the block.
-	MinTime() int64
-
-	// MaxTime returns the max time of the block.
-	MaxTime() int64
+	// Meta provides meta information about the block reader.
+	Meta() BlockMeta
 }
 
 // Appendable defines an entity to which data can be appended.

block_test.go

@@ -175,8 +175,7 @@ func TestBlockSize(t *testing.T) {
 			testutil.Ok(t, blockInit.Close())
 		}()
 		expSizeInit = blockInit.Size()
-		actSizeInit, err := testutil.DirSize(blockInit.Dir())
-		testutil.Ok(t, err)
+		actSizeInit := testutil.DirSize(t, blockInit.Dir())
 		testutil.Equals(t, expSizeInit, actSizeInit)
 	}
 
@@ -185,7 +184,7 @@ func TestBlockSize(t *testing.T) {
 		testutil.Ok(t, blockInit.Delete(1, 10, labels.NewMustRegexpMatcher("", ".*")))
 		expAfterDelete := blockInit.Size()
 		testutil.Assert(t, expAfterDelete > expSizeInit, "after a delete the block size should be bigger as the tombstone file should grow %v > %v", expAfterDelete, expSizeInit)
-		actAfterDelete, err := testutil.DirSize(blockDirInit)
+		actAfterDelete := testutil.DirSize(t, blockDirInit)
 		testutil.Ok(t, err)
 		testutil.Equals(t, expAfterDelete, actAfterDelete, "after a delete reported block size doesn't match actual disk size")
 
@@ -199,8 +198,7 @@ func TestBlockSize(t *testing.T) {
 			testutil.Ok(t, blockAfterCompact.Close())
 		}()
 		expAfterCompact := blockAfterCompact.Size()
-		actAfterCompact, err := testutil.DirSize(blockAfterCompact.Dir())
-		testutil.Ok(t, err)
+		actAfterCompact := testutil.DirSize(t, blockAfterCompact.Dir())
 		testutil.Assert(t, actAfterDelete > actAfterCompact, "after a delete and compaction the block size should be smaller %v,%v", actAfterDelete, actAfterCompact)
 		testutil.Equals(t, expAfterCompact, actAfterCompact, "after a delete and compaction reported block size doesn't match actual disk size")
 	}

cmd/tsdb/main.go

@@ -34,11 +34,19 @@ import (
 	"github.com/pkg/errors"
 	"github.com/prometheus/tsdb"
 	"github.com/prometheus/tsdb/chunks"
+	tsdb_errors "github.com/prometheus/tsdb/errors"
 	"github.com/prometheus/tsdb/labels"
 	"gopkg.in/alecthomas/kingpin.v2"
 )
 
 func main() {
+	if err := execute(); err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+}
+
+func execute() (err error) {
 	var (
 		defaultDBPath = filepath.Join("benchout", "storage")
 
@@ -61,8 +69,8 @@ func main() {
 		dumpMaxTime          = dumpCmd.Flag("max-time", "maximum timestamp to dump").Default(strconv.FormatInt(math.MaxInt64, 10)).Int64()
 	)
 
-	safeDBOptions := *tsdb.DefaultOptions
-	safeDBOptions.RetentionDuration = 0
+	logger := log.NewLogfmtLogger(log.NewSyncWriter(os.Stderr))
+	var merr tsdb_errors.MultiError
 
 	switch kingpin.MustParse(cli.Parse(os.Args[1:])) {
 	case benchWriteCmd.FullCommand():
@@ -70,21 +78,39 @@ func main() {
 			outPath:     *benchWriteOutPath,
 			numMetrics:  *benchWriteNumMetrics,
 			samplesFile: *benchSamplesFile,
+			logger:      logger,
 		}
-		wb.run()
+		return wb.run()
 	case listCmd.FullCommand():
-		db, err := tsdb.Open(*listPath, nil, nil, &safeDBOptions)
+		db, err := tsdb.OpenDBReadOnly(*listPath, nil)
 		if err != nil {
-			exitWithError(err)
+			return err
 		}
-		printBlocks(db.Blocks(), listCmdHumanReadable)
+		defer func() {
+			merr.Add(err)
+			merr.Add(db.Close())
+			err = merr.Err()
+		}()
+		blocks, err := db.Blocks()
+		if err != nil {
+			return err
+		}
+		printBlocks(blocks, listCmdHumanReadable)
 	case analyzeCmd.FullCommand():
-		db, err := tsdb.Open(*analyzePath, nil, nil, &safeDBOptions)
+		db, err := tsdb.OpenDBReadOnly(*analyzePath, nil)
 		if err != nil {
-			exitWithError(err)
+			return err
 		}
-		blocks := db.Blocks()
-		var block *tsdb.Block
+		defer func() {
+			merr.Add(err)
+			merr.Add(db.Close())
+			err = merr.Err()
+		}()
+		blocks, err := db.Blocks()
+		if err != nil {
+			return err
+		}
+		var block tsdb.BlockReader
 		if *analyzeBlockID != "" {
 			for _, b := range blocks {
 				if b.Meta().ULID.String() == *analyzeBlockID {
@@ -96,16 +122,22 @@ func main() {
 			block = blocks[len(blocks)-1]
 		}
 		if block == nil {
-			exitWithError(fmt.Errorf("block not found"))
+			return fmt.Errorf("block not found")
 		}
-		analyzeBlock(block, *analyzeLimit)
+		return analyzeBlock(block, *analyzeLimit)
 	case dumpCmd.FullCommand():
-		db, err := tsdb.Open(*dumpPath, nil, nil, &safeDBOptions)
+		db, err := tsdb.OpenDBReadOnly(*dumpPath, nil)
 		if err != nil {
-			exitWithError(err)
+			return err
 		}
-		dumpSamples(db, *dumpMinTime, *dumpMaxTime)
+		defer func() {
+			merr.Add(err)
+			merr.Add(db.Close())
+			err = merr.Err()
+		}()
+		return dumpSamples(db, *dumpMinTime, *dumpMaxTime)
 	}
+	return nil
 }
 
 type writeBenchmark struct {
@@ -120,74 +152,87 @@ type writeBenchmark struct {
 	memprof   *os.File
 	blockprof *os.File
 	mtxprof   *os.File
+	logger    log.Logger
 }
 
-func (b *writeBenchmark) run() {
+func (b *writeBenchmark) run() error {
 	if b.outPath == "" {
 		dir, err := ioutil.TempDir("", "tsdb_bench")
 		if err != nil {
-			exitWithError(err)
+			return err
 		}
 		b.outPath = dir
 		b.cleanup = true
 	}
 	if err := os.RemoveAll(b.outPath); err != nil {
-		exitWithError(err)
+		return err
 	}
 	if err := os.MkdirAll(b.outPath, 0777); err != nil {
-		exitWithError(err)
+		return err
 	}
 
 	dir := filepath.Join(b.outPath, "storage")
 
-	l := log.NewLogfmtLogger(log.NewSyncWriter(os.Stderr))
-	l = log.With(l, "ts", log.DefaultTimestampUTC, "caller", log.DefaultCaller)
+	l := log.With(b.logger, "ts", log.DefaultTimestampUTC, "caller", log.DefaultCaller)
 
 	st, err := tsdb.Open(dir, l, nil, &tsdb.Options{
 		RetentionDuration: 15 * 24 * 60 * 60 * 1000, // 15 days in milliseconds
 		BlockRanges:       tsdb.ExponentialBlockRanges(2*60*60*1000, 5, 3),
 	})
 	if err != nil {
-		exitWithError(err)
+		return err
 	}
 	b.storage = st
 
 	var labels []labels.Labels
 
-	measureTime("readData", func() {
+	_, err = measureTime("readData", func() error {
 		f, err := os.Open(b.samplesFile)
 		if err != nil {
-			exitWithError(err)
+			return err
 		}
 		defer f.Close()
 
 		labels, err = readPrometheusLabels(f, b.numMetrics)
 		if err != nil {
-			exitWithError(err)
+			return err
 		}
+		return nil
 	})
+	if err != nil {
+		return err
+	}
 
 	var total uint64
 
-	dur := measureTime("ingestScrapes", func() {
+	dur, err := measureTime("ingestScrapes", func() error {
 		b.startProfiling()
 		total, err = b.ingestScrapes(labels, 3000)
 		if err != nil {
-			exitWithError(err)
+			return err
 		}
+		return nil
 	})
+	if err != nil {
+		return err
+	}
 
 	fmt.Println(" > total samples:", total)
 	fmt.Println(" > samples/sec:", float64(total)/dur.Seconds())
 
-	measureTime("stopStorage", func() {
+	_, err = measureTime("stopStorage", func() error {
 		if err := b.storage.Close(); err != nil {
-			exitWithError(err)
+			return err
 		}
 		if err := b.stopProfiling(); err != nil {
-			exitWithError(err)
+			return err
 		}
+		return nil
 	})
+	if err != nil {
+		return err
+	}
+	return nil
 }
 
 const timeDelta = 30000
@@ -281,37 +326,38 @@ func (b *writeBenchmark) ingestScrapesShard(lbls []labels.Labels, scrapeCount in
 	return total, nil
 }
 
-func (b *writeBenchmark) startProfiling() {
+func (b *writeBenchmark) startProfiling() error {
 	var err error
 
 	// Start CPU profiling.
 	b.cpuprof, err = os.Create(filepath.Join(b.outPath, "cpu.prof"))
 	if err != nil {
-		exitWithError(fmt.Errorf("bench: could not create cpu profile: %v", err))
+		return fmt.Errorf("bench: could not create cpu profile: %v", err)
 	}
 	if err := pprof.StartCPUProfile(b.cpuprof); err != nil {
-		exitWithError(fmt.Errorf("bench: could not start CPU profile: %v", err))
+		return fmt.Errorf("bench: could not start CPU profile: %v", err)
 	}
 
 	// Start memory profiling.
 	b.memprof, err = os.Create(filepath.Join(b.outPath, "mem.prof"))
 	if err != nil {
-		exitWithError(fmt.Errorf("bench: could not create memory profile: %v", err))
+		return fmt.Errorf("bench: could not create memory profile: %v", err)
 	}
 	runtime.MemProfileRate = 64 * 1024
 
 	// Start fatal profiling.
 	b.blockprof, err = os.Create(filepath.Join(b.outPath, "block.prof"))
 	if err != nil {
-		exitWithError(fmt.Errorf("bench: could not create block profile: %v", err))
+		return fmt.Errorf("bench: could not create block profile: %v", err)
 	}
 	runtime.SetBlockProfileRate(20)
 
 	b.mtxprof, err = os.Create(filepath.Join(b.outPath, "mutex.prof"))
 	if err != nil {
-		exitWithError(fmt.Errorf("bench: could not create mutex profile: %v", err))
+		return fmt.Errorf("bench: could not create mutex profile: %v", err)
 	}
 	runtime.SetMutexProfileFraction(20)
+	return nil
 }
 
 func (b *writeBenchmark) stopProfiling() error {
@@ -346,12 +392,15 @@ func (b *writeBenchmark) stopProfiling() error {
 	return nil
 }
 
-func measureTime(stage string, f func()) time.Duration {
+func measureTime(stage string, f func() error) (time.Duration, error) {
 	fmt.Printf(">> start stage=%s\n", stage)
 	start := time.Now()
-	f()
+	err := f()
+	if err != nil {
+		return 0, err
+	}
 	fmt.Printf(">> completed stage=%s duration=%s\n", stage, time.Since(start))
-	return time.Since(start)
+	return time.Since(start), nil
 }
 
 func readPrometheusLabels(r io.Reader, n int) ([]labels.Labels, error) {
@@ -385,12 +434,7 @@ func readPrometheusLabels(r io.Reader, n int) ([]labels.Labels, error) {
 	return mets, nil
 }
 
-func exitWithError(err error) {
-	fmt.Fprintln(os.Stderr, err)
-	os.Exit(1)
-}
-
-func printBlocks(blocks []*tsdb.Block, humanReadable *bool) {
+func printBlocks(blocks []tsdb.BlockReader, humanReadable *bool) {
 	tw := tabwriter.NewWriter(os.Stdout, 0, 0, 2, ' ', 0)
 	defer tw.Flush()
 
@@ -417,21 +461,21 @@ func getFormatedTime(timestamp int64, humanReadable *bool) string {
 	return strconv.FormatInt(timestamp, 10)
 }
 
-func analyzeBlock(b *tsdb.Block, limit int) {
-	fmt.Printf("Block path: %s\n", b.Dir())
+func analyzeBlock(b tsdb.BlockReader, limit int) error {
 	meta := b.Meta()
+	fmt.Printf("Block ID: %s\n", meta.ULID)
 	// Presume 1ms resolution that Prometheus uses.
 	fmt.Printf("Duration: %s\n", (time.Duration(meta.MaxTime-meta.MinTime) * 1e6).String())
 	fmt.Printf("Series: %d\n", meta.Stats.NumSeries)
 	ir, err := b.Index()
 	if err != nil {
-		exitWithError(err)
+		return err
 	}
 	defer ir.Close()
 
 	allLabelNames, err := ir.LabelNames()
 	if err != nil {
-		exitWithError(err)
+		return err
 	}
 	fmt.Printf("Label names: %d\n", len(allLabelNames))
 
@@ -458,13 +502,13 @@ func analyzeBlock(b *tsdb.Block, limit int) {
 	entries := 0
 	p, err := ir.Postings("", "") // The special all key.
 	if err != nil {
-		exitWithError(err)
+		return err
 	}
 	lbls := labels.Labels{}
 	chks := []chunks.Meta{}
 	for p.Next() {
 		if err = ir.Series(p.At(), &lbls, &chks); err != nil {
-			exitWithError(err)
+			return err
 		}
 		// Amount of the block time range not covered by this series.
 		uncovered := uint64(meta.MaxTime-meta.MinTime) - uint64(chks[len(chks)-1].MaxTime-chks[0].MinTime)
@@ -477,7 +521,7 @@ func analyzeBlock(b *tsdb.Block, limit int) {
 		}
 	}
 	if p.Err() != nil {
-		exitWithError(p.Err())
+		return p.Err()
 	}
 	fmt.Printf("Postings (unique label pairs): %d\n", len(labelpairsUncovered))
 	fmt.Printf("Postings entries (total label pairs): %d\n", entries)
@@ -510,14 +554,14 @@ func analyzeBlock(b *tsdb.Block, limit int) {
 	for _, n := range allLabelNames {
 		values, err := ir.LabelValues(n)
 		if err != nil {
-			exitWithError(err)
+			return err
 		}
 		var cumulativeLength uint64
 
 		for i := 0; i < values.Len(); i++ {
 			value, _ := values.At(i)
 			if err != nil {
-				exitWithError(err)
+				return err
 			}
 			for _, str := range value {
 				cumulativeLength += uint64(len(str))
@@ -534,7 +578,7 @@ func analyzeBlock(b *tsdb.Block, limit int) {
 	for _, n := range allLabelNames {
 		lv, err := ir.LabelValues(n)
 		if err != nil {
-			exitWithError(err)
+			return err
 		}
 		postingInfos = append(postingInfos, postingInfo{n, uint64(lv.Len())})
 	}
@@ -544,41 +588,49 @@ func analyzeBlock(b *tsdb.Block, limit int) {
 	postingInfos = postingInfos[:0]
 	lv, err := ir.LabelValues("__name__")
 	if err != nil {
-		exitWithError(err)
+		return err
 	}
 	for i := 0; i < lv.Len(); i++ {
 		names, err := lv.At(i)
 		if err != nil {
-			exitWithError(err)
+			return err
 		}
 		for _, n := range names {
 			postings, err := ir.Postings("__name__", n)
 			if err != nil {
-				exitWithError(err)
+				return err
 			}
 			count := 0
 			for postings.Next() {
 				count++
 			}
 			if postings.Err() != nil {
-				exitWithError(postings.Err())
+				return postings.Err()
 			}
 			postingInfos = append(postingInfos, postingInfo{n, uint64(count)})
 		}
 	}
 	fmt.Printf("\nHighest cardinality metric names:\n")
 	printInfo(postingInfos)
+	return nil
 }
 
-func dumpSamples(db *tsdb.DB, mint, maxt int64) {
+func dumpSamples(db *tsdb.DBReadOnly, mint, maxt int64) (err error) {
+
 	q, err := db.Querier(mint, maxt)
 	if err != nil {
-		exitWithError(err)
+		return err
 	}
+	defer func() {
+		var merr tsdb_errors.MultiError
+		merr.Add(err)
+		merr.Add(q.Close())
+		err = merr.Err()
+	}()
 
 	ss, err := q.Select(labels.NewMustRegexpMatcher("", ".*"))
 	if err != nil {
-		exitWithError(err)
+		return err
 	}
 
 	for ss.Next() {
@@ -590,11 +642,12 @@ func dumpSamples(db *tsdb.DB, mint, maxt int64) {
 			fmt.Printf("%s %g %d\n", labels, val, ts)
 		}
 		if it.Err() != nil {
-			exitWithError(ss.Err())
+			return ss.Err()
 		}
 	}
 
 	if ss.Err() != nil {
-		exitWithError(ss.Err())
+		return ss.Err()
 	}
+	return nil
 }

compact.go

@@ -662,7 +662,7 @@ func (c *LeveledCompactor) populateBlock(blocks []BlockReader, meta *BlockMeta,
 	}()
 	c.metrics.populatingBlocks.Set(1)
 
-	globalMaxt := blocks[0].MaxTime()
+	globalMaxt := blocks[0].Meta().MaxTime
 	for i, b := range blocks {
 		select {
 		case <-c.ctx.Done():
@@ -671,13 +671,13 @@ func (c *LeveledCompactor) populateBlock(blocks []BlockReader, meta *BlockMeta,
 		}
 
 		if !overlapping {
-			if i > 0 && b.MinTime() < globalMaxt {
+			if i > 0 && b.Meta().MinTime < globalMaxt {
 				c.metrics.overlappingBlocks.Inc()
 				overlapping = true
 				level.Warn(c.logger).Log("msg", "found overlapping blocks during compaction", "ulid", meta.ULID)
 			}
-			if b.MaxTime() > globalMaxt {
-				globalMaxt = b.MaxTime()
+			if b.Meta().MaxTime > globalMaxt {
+				globalMaxt = b.Meta().MaxTime
 			}
 		}
 

compact_test.go

@@ -458,8 +458,7 @@ type erringBReader struct{}
 func (erringBReader) Index() (IndexReader, error)          { return nil, errors.New("index") }
 func (erringBReader) Chunks() (ChunkReader, error)         { return nil, errors.New("chunks") }
 func (erringBReader) Tombstones() (TombstoneReader, error) { return nil, errors.New("tombstones") }
-func (erringBReader) MinTime() int64                       { return 0 }
-func (erringBReader) MaxTime() int64                       { return 0 }
+func (erringBReader) Meta() BlockMeta                      { return BlockMeta{} }
 
 type nopChunkWriter struct{}
 

db.go

@@ -250,6 +250,178 @@ func newDBMetrics(db *DB, r prometheus.Registerer) *dbMetrics {
 	return m
 }
 
+// ErrClosed is returned when the db is closed.
+var ErrClosed = errors.New("db already closed")
+
+// DBReadOnly provides APIs for read only operations on a database.
+// Current implementation doesn't support concurency so
+// all API calls should happen in the same go routine.
+type DBReadOnly struct {
+	logger  log.Logger
+	dir     string
+	closers []io.Closer
+	closed  chan struct{}
+}
+
+// OpenDBReadOnly opens DB in the given directory for read only operations.
+func OpenDBReadOnly(dir string, l log.Logger) (*DBReadOnly, error) {
+	if _, err := os.Stat(dir); err != nil {
+		return nil, errors.Wrap(err, "openning the db dir")
+	}
+
+	if l == nil {
+		l = log.NewNopLogger()
+	}
+
+	return &DBReadOnly{
+		logger: l,
+		dir:    dir,
+		closed: make(chan struct{}),
+	}, nil
+}
+
+// Querier loads the wal and returns a new querier over the data partition for the given time range.
+// Current implementation doesn't support multiple Queriers.
+func (db *DBReadOnly) Querier(mint, maxt int64) (Querier, error) {
+	select {
+	case <-db.closed:
+		return nil, ErrClosed
+	default:
+	}
+	blocksReaders, err := db.Blocks()
+	if err != nil {
+		return nil, err
+	}
+	blocks := make([]*Block, len(blocksReaders))
+	for i, b := range blocksReaders {
+		b, ok := b.(*Block)
+		if !ok {
+			return nil, errors.New("unable to convert a read only block to a normal block")
+		}
+		blocks[i] = b
+	}
+
+	head, err := NewHead(nil, db.logger, nil, 1)
+	if err != nil {
+		return nil, err
+	}
+	maxBlockTime := int64(math.MinInt64)
+	if len(blocks) > 0 {
+		maxBlockTime = blocks[len(blocks)-1].Meta().MaxTime
+	}
+
+	// Also add the WAL if the current blocks don't cover the requestes time range.
+	if maxBlockTime <= maxt {
+		w, err := wal.Open(db.logger, nil, filepath.Join(db.dir, "wal"))
+		if err != nil {
+			return nil, err
+		}
+		head, err = NewHead(nil, db.logger, w, 1)
+		if err != nil {
+			return nil, err
+		}
+		// Set the min valid time for the ingested wal samples
+		// to be no lower than the maxt of the last block.
+		if err := head.Init(maxBlockTime); err != nil {
+			return nil, errors.Wrap(err, "read WAL")
+		}
+		// Set the wal to nil to disable all wal operations.
+		// This is mainly to avoid blocking when closing the head.
+		head.wal = nil
+
+		db.closers = append(db.closers, head)
+	}
+
+	// TODO: Refactor so that it is possible to obtain a Querier without initializing a writable DB instance.
+	// Option 1: refactor DB to have the Querier implementation using the DBReadOnly.Querier implementation not the opposite.
+	// Option 2: refactor Querier to use another independent func which
+	// can than be used by a read only and writable db instances without any code duplication.
+	dbWritable := &DB{
+		dir:    db.dir,
+		logger: db.logger,
+		blocks: blocks,
+		head:   head,
+	}
+
+	return dbWritable.Querier(mint, maxt)
+}
+
+// Blocks returns a slice of block readers for persisted blocks.
+func (db *DBReadOnly) Blocks() ([]BlockReader, error) {
+	select {
+	case <-db.closed:
+		return nil, ErrClosed
+	default:
+	}
+	loadable, corrupted, err := openBlocks(db.logger, db.dir, nil, nil)
+	if err != nil {
+		return nil, err
+	}
+
+	// Corrupted blocks that have been superseded by a loadable block can be safely ignored.
+	for _, block := range loadable {
+		for _, b := range block.Meta().Compaction.Parents {
+			delete(corrupted, b.ULID)
+		}
+	}
+	if len(corrupted) > 0 {
+		for _, b := range loadable {
+			if err := b.Close(); err != nil {
+				level.Warn(db.logger).Log("msg", "closing a block", err)
+			}
+		}
+		return nil, errors.Errorf("unexpected corrupted block:%v", corrupted)
+	}
+
+	if len(loadable) == 0 {
+		return nil, errors.New("no blocks found")
+	}
+
+	sort.Slice(loadable, func(i, j int) bool {
+		return loadable[i].Meta().MinTime < loadable[j].Meta().MinTime
+	})
+
+	blockMetas := make([]BlockMeta, 0, len(loadable))
+	for _, b := range loadable {
+		blockMetas = append(blockMetas, b.Meta())
+	}
+	if overlaps := OverlappingBlocks(blockMetas); len(overlaps) > 0 {
+		level.Warn(db.logger).Log("msg", "overlapping blocks found during opening", "detail", overlaps.String())
+	}
+
+	// Close all previously open readers and add the new ones to the cache.
+	for _, closer := range db.closers {
+		closer.Close()
+	}
+
+	blockClosers := make([]io.Closer, len(loadable))
+	blockReaders := make([]BlockReader, len(loadable))
+	for i, b := range loadable {
+		blockClosers[i] = b
+		blockReaders[i] = b
+	}
+	db.closers = blockClosers
+
+	return blockReaders, nil
+}
+
+// Close all block readers.
+func (db *DBReadOnly) Close() error {
+	select {
+	case <-db.closed:
+		return ErrClosed
+	default:
+	}
+	close(db.closed)
+
+	var merr tsdb_errors.MultiError
+
+	for _, b := range db.closers {
+		merr.Add(b.Close())
+	}
+	return merr.Err()
+}
+
 // Open returns a new DB in the given directory.
 func Open(dir string, l log.Logger, r prometheus.Registerer, opts *Options) (db *DB, err error) {
 	if err := os.MkdirAll(dir, 0777); err != nil {
@@ -514,8 +686,10 @@ func (db *DB) compact() (err error) {
 	return nil
 }
 
-func (db *DB) getBlock(id ulid.ULID) (*Block, bool) {
-	for _, b := range db.blocks {
+// getBlock iterates a given block range to find a block by a given id.
+// If found it returns the block itself and a boolean to indicate that it was found.
+func getBlock(allBlocks []*Block, id ulid.ULID) (*Block, bool) {
+	for _, b := range allBlocks {
 		if b.Meta().ULID == id {
 			return b, true
 		}
@@ -533,14 +707,14 @@ func (db *DB) reload() (err error) {
 		db.metrics.reloads.Inc()
 	}()
 
-	loadable, corrupted, err := db.openBlocks()
+	loadable, corrupted, err := openBlocks(db.logger, db.dir, db.blocks, db.chunkPool)
 	if err != nil {
 		return err
 	}
 
 	deletable := db.deletableBlocks(loadable)
 
-	// Corrupted blocks that have been replaced by parents can be safely ignored and deleted.
+	// Corrupted blocks that have been superseded by a loadable block can be safely ignored.
 	// This makes it resilient against the process crashing towards the end of a compaction.
 	// Creation of a new block and deletion of its parents cannot happen atomically.
 	// By creating blocks with their parents, we can pick up the deletion where it left off during a crash.
@@ -553,7 +727,7 @@ func (db *DB) reload() (err error) {
 	if len(corrupted) > 0 {
 		// Close all new blocks to release the lock for windows.
 		for _, block := range loadable {
-			if _, loaded := db.getBlock(block.Meta().ULID); !loaded {
+			if _, open := getBlock(db.blocks, block.Meta().ULID); !open {
 				block.Close()
 			}
 		}
@@ -621,24 +795,24 @@ func (db *DB) reload() (err error) {
 	return errors.Wrap(db.head.Truncate(maxt), "head truncate failed")
 }
 
-func (db *DB) openBlocks() (blocks []*Block, corrupted map[ulid.ULID]error, err error) {
-	dirs, err := blockDirs(db.dir)
+func openBlocks(l log.Logger, dir string, loaded []*Block, chunkPool chunkenc.Pool) (blocks []*Block, corrupted map[ulid.ULID]error, err error) {
+	bDirs, err := blockDirs(dir)
 	if err != nil {
 		return nil, nil, errors.Wrap(err, "find blocks")
 	}
 
 	corrupted = make(map[ulid.ULID]error)
-	for _, dir := range dirs {
-		meta, _, err := readMetaFile(dir)
+	for _, bDir := range bDirs {
+		meta, _, err := readMetaFile(bDir)
 		if err != nil {
-			level.Error(db.logger).Log("msg", "not a block dir", "dir", dir)
+			level.Error(l).Log("msg", "not a block dir", "dir", bDir)
 			continue
 		}
 
 		// See if we already have the block in memory or open it otherwise.
-		block, ok := db.getBlock(meta.ULID)
-		if !ok {
-			block, err = OpenBlock(db.logger, dir, db.chunkPool)
+		block, open := getBlock(loaded, meta.ULID)
+		if !open {
+			block, err = OpenBlock(l, bDir, chunkPool)
 			if err != nil {
 				corrupted[meta.ULID] = err
 				continue

db_test.go

@@ -1113,8 +1113,7 @@ func TestSizeRetention(t *testing.T) {
 	testutil.Ok(t, db.reload())                                       // Reload the db to register the new db size.
 	testutil.Equals(t, len(blocks), len(db.Blocks()))                 // Ensure all blocks are registered.
 	expSize := int64(prom_testutil.ToFloat64(db.metrics.blocksBytes)) // Use the the actual internal metrics.
-	actSize, err := testutil.DirSize(db.Dir())
-	testutil.Ok(t, err)
+	actSize := testutil.DirSize(t, db.Dir())
 	testutil.Equals(t, expSize, actSize, "registered size doesn't match actual disk size")
 
 	// Decrease the max bytes limit so that a delete is triggered.
@@ -1128,8 +1127,7 @@ func TestSizeRetention(t *testing.T) {
 	actBlocks := db.Blocks()
 	expSize = int64(prom_testutil.ToFloat64(db.metrics.blocksBytes))
 	actRetentCount := int(prom_testutil.ToFloat64(db.metrics.sizeRetentionCount))
-	actSize, err = testutil.DirSize(db.Dir())
-	testutil.Ok(t, err)
+	actSize = testutil.DirSize(t, db.Dir())
 
 	testutil.Equals(t, 1, actRetentCount, "metric retention count mismatch")
 	testutil.Equals(t, actSize, expSize, "metric db size doesn't match actual disk size")
@@ -2232,3 +2230,108 @@ func TestBlockRanges(t *testing.T) {
 		t.Fatalf("new block overlaps  old:%v,new:%v", db.Blocks()[2].Meta(), db.Blocks()[3].Meta())
 	}
 }
+
+// TestDBReadOnly ensures that opening a DB in readonly mode doesn't modify any files on the disk.
+// It also checks that the API calls return equivalent results as a normal db.Open() mode.
+func TestDBReadOnly(t *testing.T) {
+	var (
+		dbDir          string
+		logger         = log.NewLogfmtLogger(log.NewSyncWriter(os.Stderr))
+		expBlocks      []*Block
+		expSeries      map[string][]tsdbutil.Sample
+		expSeriesCount int
+		expDBHash      []byte
+		matchAll       = labels.NewEqualMatcher("", "")
+		err            error
+	)
+
+	// Boostrap the db.
+	{
+		dbDir, err = ioutil.TempDir("", "test")
+		testutil.Ok(t, err)
+
+		defer func() {
+			testutil.Ok(t, os.RemoveAll(dbDir))
+		}()
+
+		dbBlocks := []*BlockMeta{
+			{MinTime: 10, MaxTime: 11},
+			{MinTime: 11, MaxTime: 12},
+			{MinTime: 12, MaxTime: 13},
+		}
+
+		for _, m := range dbBlocks {
+			createBlock(t, dbDir, genSeries(1, 1, m.MinTime, m.MaxTime))
+		}
+		expSeriesCount++
+	}
+
+	// Open a normal db to use for a comparison.
+	{
+		dbWritable, err := Open(dbDir, logger, nil, nil)
+		testutil.Ok(t, err)
+		dbWritable.DisableCompactions()
+
+		dbSizeBeforeAppend := testutil.DirSize(t, dbWritable.Dir())
+		app := dbWritable.Appender()
+		_, err = app.Add(labels.FromStrings("foo", "bar"), dbWritable.Head().MaxTime()+1, 0)
+		testutil.Ok(t, err)
+		testutil.Ok(t, app.Commit())
+		expSeriesCount++
+
+		expBlocks = dbWritable.Blocks()
+		expDbSize := testutil.DirSize(t, dbWritable.Dir())
+		testutil.Assert(t, expDbSize > dbSizeBeforeAppend, "db size didn't increase after an append")
+
+		q, err := dbWritable.Querier(math.MinInt64, math.MaxInt64)
+		testutil.Ok(t, err)
+		expSeries = query(t, q, matchAll)
+
+		testutil.Ok(t, dbWritable.Close()) // Close here to allow getting the dir hash for windows.
+		expDBHash = testutil.DirHash(t, dbWritable.Dir())
+	}
+
+	// Open a read only db and ensure that the API returns the same result as the normal DB.
+	{
+		dbReadOnly, err := OpenDBReadOnly(dbDir, logger)
+		testutil.Ok(t, err)
+		defer func() {
+			testutil.Ok(t, dbReadOnly.Close())
+		}()
+		blocks, err := dbReadOnly.Blocks()
+		testutil.Ok(t, err)
+		testutil.Equals(t, len(expBlocks), len(blocks))
+
+		for i, expBlock := range expBlocks {
+			testutil.Equals(t, expBlock.Meta(), blocks[i].Meta(), "block meta mismatch")
+		}
+
+		q, err := dbReadOnly.Querier(math.MinInt64, math.MaxInt64)
+		testutil.Ok(t, err)
+		readOnlySeries := query(t, q, matchAll)
+		readOnlyDBHash := testutil.DirHash(t, dbDir)
+
+		testutil.Equals(t, expSeriesCount, len(readOnlySeries), "total series mismatch")
+		testutil.Equals(t, expSeries, readOnlySeries, "series mismatch")
+		testutil.Equals(t, expDBHash, readOnlyDBHash, "after all read operations the db hash should remain the same")
+	}
+}
+
+// TestDBReadOnlyClosing ensures that after closing the db
+// all api methods return an ErrClosed.
+func TestDBReadOnlyClosing(t *testing.T) {
+	dbDir, err := ioutil.TempDir("", "test")
+	testutil.Ok(t, err)
+
+	defer func() {
+		testutil.Ok(t, os.RemoveAll(dbDir))
+	}()
+	db, err := OpenDBReadOnly(dbDir, log.NewLogfmtLogger(log.NewSyncWriter(os.Stderr)))
+	testutil.Ok(t, err)
+	testutil.Ok(t, db.Close())
+	testutil.Equals(t, db.Close(), ErrClosed)
+	_, err = db.Blocks()
+	testutil.Equals(t, err, ErrClosed)
+	_, err = db.Querier(0, 1)
+	testutil.Equals(t, err, ErrClosed)
+}

head.go

@@ -25,6 +25,7 @@ import (
 
 	"github.com/go-kit/kit/log"
 	"github.com/go-kit/kit/log/level"
+	"github.com/oklog/ulid"
 	"github.com/pkg/errors"
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/tsdb/chunkenc"
@@ -64,6 +65,7 @@ type Head struct {
 	logger     log.Logger
 	appendPool sync.Pool
 	bytesPool  sync.Pool
+	numSeries  uint64
 
 	minTime, maxTime int64 // Current min and max of the samples included in the head.
 	minValidTime     int64 // Mint allowed to be added to the head. It shouldn't be lower than the maxt of the last persisted block.
@@ -84,7 +86,7 @@ type Head struct {
 
 type headMetrics struct {
 	activeAppenders         prometheus.Gauge
-	series                  prometheus.Gauge
+	series                  prometheus.GaugeFunc
 	seriesCreated           prometheus.Counter
 	seriesRemoved           prometheus.Counter
 	seriesNotFound          prometheus.Counter
@@ -112,9 +114,11 @@ func newHeadMetrics(h *Head, r prometheus.Registerer) *headMetrics {
 		Name: "prometheus_tsdb_head_active_appenders",
 		Help: "Number of currently active appender transactions",
 	})
-	m.series = prometheus.NewGauge(prometheus.GaugeOpts{
+	m.series = prometheus.NewGaugeFunc(prometheus.GaugeOpts{
 		Name: "prometheus_tsdb_head_series",
 		Help: "Total number of series in the head block.",
+	}, func() float64 {
+		return float64(h.NumSeries())
 	})
 	m.seriesCreated = prometheus.NewCounter(prometheus.CounterOpts{
 		Name: "prometheus_tsdb_head_series_created_total",
@@ -701,6 +705,21 @@ func (h *rangeHead) MaxTime() int64 {
 	return h.maxt
 }
 
+func (h *rangeHead) NumSeries() uint64 {
+	return h.head.NumSeries()
+}
+
+func (h *rangeHead) Meta() BlockMeta {
+	return BlockMeta{
+		MinTime: h.MinTime(),
+		MaxTime: h.MaxTime(),
+		ULID:    h.head.Meta().ULID,
+		Stats: BlockStats{
+			NumSeries: h.NumSeries(),
+		},
+	}
+}
+
 // initAppender is a helper to initialize the time bounds of the head
 // upon the first sample it receives.
 type initAppender struct {
@@ -1025,9 +1044,11 @@ func (h *Head) gc() {
 	seriesRemoved := len(deleted)
 
 	h.metrics.seriesRemoved.Add(float64(seriesRemoved))
-	h.metrics.series.Sub(float64(seriesRemoved))
 	h.metrics.chunksRemoved.Add(float64(chunksRemoved))
 	h.metrics.chunks.Sub(float64(chunksRemoved))
+	// Using AddUint64 to substract series removed.
+	// See: https://golang.org/pkg/sync/atomic/#AddUint64.
+	atomic.AddUint64(&h.numSeries, ^uint64(seriesRemoved-1))
 
 	// Remove deleted series IDs from the postings lists.
 	h.postings.Delete(deleted)
@@ -1104,6 +1125,26 @@ func (h *Head) chunksRange(mint, maxt int64) *headChunkReader {
 	return &headChunkReader{head: h, mint: mint, maxt: maxt}
 }
 
+// NumSeries returns the number of active series in the head.
+func (h *Head) NumSeries() uint64 {
+	return atomic.LoadUint64(&h.numSeries)
+}
+
+// Meta returns meta information about the head.
+// The head is dynamic so will return dynamic results.
+func (h *Head) Meta() BlockMeta {
+	var id [16]byte
+	copy(id[:], "______head______")
+	return BlockMeta{
+		MinTime: h.MinTime(),
+		MaxTime: h.MaxTime(),
+		ULID:    ulid.ULID(id),
+		Stats: BlockStats{
+			NumSeries: h.NumSeries(),
+		},
+	}
+}
+
 // MinTime returns the lowest time bound on visible data in the head.
 func (h *Head) MinTime() int64 {
 	return atomic.LoadInt64(&h.minTime)
@@ -1350,8 +1391,8 @@ func (h *Head) getOrCreateWithID(id, hash uint64, lset labels.Labels) (*memSerie
 		return s, false
 	}
 
-	h.metrics.series.Inc()
 	h.metrics.seriesCreated.Inc()
+	atomic.AddUint64(&h.numSeries, 1)
 
 	h.postings.Add(id, lset)
 

mocks_test.go

@@ -75,5 +75,4 @@ type mockBReader struct {
 func (r *mockBReader) Index() (IndexReader, error)          { return r.ir, nil }
 func (r *mockBReader) Chunks() (ChunkReader, error)         { return r.cr, nil }
 func (r *mockBReader) Tombstones() (TombstoneReader, error) { return newMemTombstones(), nil }
-func (r *mockBReader) MinTime() int64                       { return r.mint }
-func (r *mockBReader) MaxTime() int64                       { return r.maxt }
+func (r *mockBReader) Meta() BlockMeta                      { return BlockMeta{MinTime: r.mint, MaxTime: r.maxt} }

testutil/directory.go

@@ -14,9 +14,13 @@
 package testutil
 
 import (
+	"crypto/sha256"
+	"io"
 	"io/ioutil"
 	"os"
 	"path/filepath"
+	"strconv"
+	"testing"
 )
 
 const (
@@ -130,16 +134,49 @@ func NewTemporaryDirectory(name string, t T) (handler TemporaryDirectory) {
 }
 
 // DirSize returns the size in bytes of all files in a directory.
-func DirSize(path string) (int64, error) {
+func DirSize(t *testing.T, path string) int64 {
 	var size int64
 	err := filepath.Walk(path, func(_ string, info os.FileInfo, err error) error {
-		if err != nil {
-			return err
-		}
+		Ok(t, err)
 		if !info.IsDir() {
 			size += info.Size()
 		}
 		return nil
 	})
-	return size, err
+	Ok(t, err)
+	return size
+}
+
+// DirHash returns a hash of all files attribites and their content within a directory.
+func DirHash(t *testing.T, path string) []byte {
+	hash := sha256.New()
+	err := filepath.Walk(path, func(path string, info os.FileInfo, err error) error {
+		Ok(t, err)
+
+		if info.IsDir() {
+			return nil
+		}
+		f, err := os.Open(path)
+		Ok(t, err)
+		defer f.Close()
+
+		_, err = io.Copy(hash, f)
+		Ok(t, err)
+
+		_, err = io.WriteString(hash, strconv.Itoa(int(info.Size())))
+		Ok(t, err)
+
+		_, err = io.WriteString(hash, info.Name())
+		Ok(t, err)
+
+		modTime, err := info.ModTime().GobEncode()
+		Ok(t, err)
+
+		_, err = io.WriteString(hash, string(modTime))
+		Ok(t, err)
+		return nil
+	})
+	Ok(t, err)
+
+	return hash.Sum(nil)
 }

wal/wal.go

@@ -203,6 +203,48 @@ func NewSize(logger log.Logger, reg prometheus.Registerer, dir string, segmentSi
 		stopc:       make(chan chan struct{}),
 		compress:    compress,
 	}
+	registerMetrics(reg, w)
+
+	_, j, err := w.Segments()
+	// Index of the Segment we want to open and write to.
+	writeSegmentIndex := 0
+	if err != nil {
+		return nil, errors.Wrap(err, "get segment range")
+	}
+	// If some segments already exist create one with a higher index than the last segment.
+	if j != -1 {
+		writeSegmentIndex = j + 1
+	}
+
+	segment, err := CreateSegment(w.dir, writeSegmentIndex)
+	if err != nil {
+		return nil, err
+	}
+
+	if err := w.setSegment(segment); err != nil {
+		return nil, err
+	}
+
+	go w.run()
+
+	return w, nil
+}
+
+// Open an existing WAL.
+func Open(logger log.Logger, reg prometheus.Registerer, dir string) (*WAL, error) {
+	if logger == nil {
+		logger = log.NewNopLogger()
+	}
+	w := &WAL{
+		dir:    dir,
+		logger: logger,
+	}
+
+	registerMetrics(reg, w)
+	return w, nil
+}
+
+func registerMetrics(reg prometheus.Registerer, w *WAL) {
 	w.fsyncDuration = prometheus.NewSummary(prometheus.SummaryOpts{
 		Name:       "prometheus_tsdb_wal_fsync_duration_seconds",
 		Help:       "Duration of WAL fsync.",
@@ -231,30 +273,6 @@ func NewSize(logger log.Logger, reg prometheus.Registerer, dir string, segmentSi
 	if reg != nil {
 		reg.MustRegister(w.fsyncDuration, w.pageFlushes, w.pageCompletions, w.truncateFail, w.truncateTotal, w.currentSegment)
 	}
-
-	_, j, err := w.Segments()
-	// Index of the Segment we want to open and write to.
-	writeSegmentIndex := 0
-	if err != nil {
-		return nil, errors.Wrap(err, "get segment range")
-	}
-	// If some segments already exist create one with a higher index than the last segment.
-	if j != -1 {
-		writeSegmentIndex = j + 1
-	}
-
-	segment, err := CreateSegment(w.dir, writeSegmentIndex)
-	if err != nil {
-		return nil, err
-	}
-
-	if err := w.setSegment(segment); err != nil {
-		return nil, err
-	}
-
-	go w.run()
-
-	return w, nil
 }
 
 // CompressionEnabled returns if compression is enabled on this WAL.
@@ -302,7 +320,6 @@ func (w *WAL) Repair(origErr error) error {
 	if cerr.Segment < 0 {
 		return errors.New("corruption error does not specify position")
 	}
-
 	level.Warn(w.logger).Log("msg", "starting corruption repair",
 		"segment", cerr.Segment, "offset", cerr.Offset)
 
@@ -487,7 +504,6 @@ func (w *WAL) flushPage(clear bool) error {
 
 // First Byte of header format:
 // [ 4 bits unallocated] [1 bit snappy compression flag] [ 3 bit record type ]
-
 const (
 	snappyMask  = 1 << 3
 	recTypeMask = snappyMask - 1

wal/wal_test.go

@@ -408,10 +408,8 @@ func TestCompression(t *testing.T) {
 		testutil.Ok(t, os.RemoveAll(dirUnCompressed))
 	}()
 
-	uncompressedSize, err := testutil.DirSize(dirUnCompressed)
-	testutil.Ok(t, err)
-	compressedSize, err := testutil.DirSize(dirCompressed)
-	testutil.Ok(t, err)
+	uncompressedSize := testutil.DirSize(t, dirUnCompressed)
+	compressedSize := testutil.DirSize(t, dirCompressed)
 
 	testutil.Assert(t, float64(uncompressedSize)*0.75 > float64(compressedSize), "Compressing zeroes should save at least 25%% space - uncompressedSize: %d, compressedSize: %d", uncompressedSize, compressedSize)
 }